Head-up instrument displays (HUD) for vehicles have been used to provide virtual images that appear to be located ahead of the vehicle windshield toward the front of the vehicle. These displays provide the advantage of increased safety since the operator does not have to divert attention significantly from viewing the outside to check the instrument status.
Most conventional windshields are made by placing a layer of adhesive, e.g. polyvinyl butyral (PVB) or polyurethane, which has a substantially uniform thickness between two sheets of glass and laminating the sandwiched assembly in an autoclave. In a conventional HUD where the image source is located away from the windshield, the light from the image source reflects off the windshield toward the viewer who sees the image floating in space beyond the windshield. If a conventional windshield is used, the viewer sees two separated images, one from the front surface of the windshield and one from the back surface. These ghost images interfere with acceptable viewing.
Approaches to reduce the ghost image problem include use of an adhesive interlayer disposed within the windshield with the interlayer having a predetermined wedge angle. The wedge angle is selected pursuant to the anticipated installation angle of the windshield, thickness of the glass sheets, and angle of incidence of the projected image so as to substantially superimpose the reflected images generated from a dashboard display device and reflected from the inboard surface and outboard surface of the windshield into a substantially singular image in the eyes of the vehicle operator. Among the approaches are those disclosed in U.S. Pat. 5,013,134 and Published PCT Application WO 91/06031.
The wedge angle for such devices is established by using an adhesive sheet having a wedge-shaped thickness profile extending continuously across the width from edge to edge of the sheet. Such a sheet and a process for preparing the sheet is disclosed in U.S. Pat. 5,087,502.
Web handling problems are encountered in manufacturing and transporting webs having a wedge-shaped thickness profile extending continuously across the width of the sheet. Web walking and telescoping of the wound up web occur damaging the web and rendering it unusable in preparing windshields. These problems also limits "off-line" tinting of the web. While special web handling equipment and procedures can be used, this is expensive and usually requires duplicating existing equipment used in preparing conventional adhesive sheeting. These problems may be alleviated by making only short rolls. However, short rolls necessitate frequent changing of rolls during production and by the customer who processes the sheeting into windshields.